The present invention relates to a retractor for a seat belt device, and more particularly to a seat belt retractor having an energy absorbing mechanism including a torsion bar.
A retractor for seat belt of a motor vehicle is used for restraining an occupant on a seat of the vehicle to secure a safety of the occupant. In this field of the seat belt retractor, an emergency-locking type belt retractor is used which includes an emergency locking mechanism for physically locking a retractor by means of an inertia sensing means responding to abrupt acceleration, impact, or abrupt deceleration, whereby it effectively and safely restrains the occupant.
U.S. Pat. No. 3,929,300 and U.S. Pat. No. 4,366,934, and Japanese Utility Model Examined Publication No. Hei.2-45088 may be enumerated for the conventional techniques for the emergency-locking type belt retractor. In the locking mechanism disclosed in those documents, an engaging member which is provided at one end of a take-up shaft on which a webbing is wound, in case of emergency, comes in engagement with an engaging-member receiving part of a retractor base, to thereby lock the rotation of the take-up shaft in the web draw-out direction.
When an impact force by collision is extremely large, a webbing tension increases with the elapsing of time after the collision. As a result, an abrupt deceleration acts on the occupant body, and a load acting on the occupant is extremely large. Various proposals to cope with this problem have been made. One of those proposals uses an energy absorbing mechanism which absorbs an impact force acting on the occupant body in such a manner that when a load acting on the webbing exceeds a predetermined value of load, it allows the seat belt to be drawn out a predetermined length of the seat belt. A specific example of the proposal is taught by U.S. Pat. No. 3,741,494, entitled "Energy Absorber for Automobile Safety Belt".
The energy absorber includes a take-up member (bobbin) serving as a portion through which a force is transmitted in the energy absorber, a holder (retractor base) rotatable relative to the take-up member, and a torsion bar (energy absorbing mechanism) which is located between the holder and the take-up member. When the take-up shaft is locked in case of emergency and then a load acts on the locking means, the torsion bar per se is twisted to absorb an impact energy acting on the occupant as a deformation work of the torsion bar.
The torsion bar disclosed in U.S. Pat. No. 3,741,494 follows. A torque is applied to one end of the torsion bar of which the other end is fixed, to thereby plastically deform the torsion bar. During the twisting of the torsion bar, the fixing ends as both ends of the torsion bar and the torque input terminal need hold the torsion bar by sufficiently large holding force.
Where the hardness of the torsion bar is uniform over its entire length, to secure a large holding force, the section moduli of the cross sections of both ends of the torsion bar must be set at large values correspondingly. Therefore, the necessity is to increase the size of both ends of the torsion bar, and size increase of the components parts located near the torsion bar is unavoidable. The result is that the seat belt retractor per se is increased in size.